The present invention relates to a tuner for diversity reception and, more particularly, to a tuner equipped with a receiving antenna system consisting of a fixed antenna and a retractive antenna for diversity reception wherein the antenna being placed in a stronger electric field is selected for receiving a radio signal in accordance with a variation of a received electric field strength.
In a motor vehicle radio receiver or the like, in order to eliminate a reception failure caused by shielding of an electromagnetic wave or a multipath of propagation in mobile reception, an antenna diversity reception has been performed by switching a plurality of antennas, which include a fixed antenna such as a rear window antenna and a retractive antenna such as a motor driven whip antenna, for receiving a radio signal with the antenna which is at a stronger electric field strength than the other. In such an antenna diversity reception system, the retractive antenna is normally retracted when the radio receiver is not in use and protracted to a predetermined length by motor driving when a power source of the radio receiver is turned on. Accordingly, when the power source of the radio receiver is turned off, the retractive antenna is retracted again by motor driving and enclosed in the original position.
In general, it is preferable to vary a length of the retractive antenna depending on a wave length of receiving radio signal for better reception. Consequently, an amount of protraction of the retractive antenna is controlled automatically in order to bring it to the most suitable antenna length such that not only at the time of ON/OFF operation of the power source but also at the time of changing FM/AM band and further, while receiving an FM broadcast, at the time of changing the reception from an FM station within a low frequency band (for example, within a range of 76-83 MHz) to an FM station within a high frequency band (for example, within a range of 84-90 MHz) or at the time of changing the reception from an FM station within the high frequency band to an FM station within the low frequency band.
However, if the retractive antenna is selected as a receiving antenna while it is being protracted or retracted, there has been a problem such that noise encountered in rubbing of the antenna elements during the movement thereof or noise generated by the antenna driven motor interferes with the reception of the radio signal by a radio receiver.
It is therefore an object of this invention to solve such a problem in the prior art and to provide a radio tuner for diversity reception free from noise due to the movement of antenna elements for protracting or retracting a length of a retractive antenna.